This invention relates to a method for cleaning scale and stains from the interior surfaces of a water-containing system having a reservoir, auxiliary equipment and piping without having to drain the water from the system. Scale and stains are objectionable because they can be unattractive, particularly in swimming pools and spas, and lead to the coating and plugging of lines, filters, pumps, heaters and heat exchangers in the system thereby reducing their efficiency and effectiveness. Scale can also lead to microbiological havens and to undesirable corrosion of surfaces which would eventually require replacement of the corroded part. In all cases it is desirable to maintain the water-contact surfaces of the system in a scale and stain free condition.
Common practice has been to drain the water from the system and then physically scrape and/or wash the scale and stain from the surface with a strong mineral acid. This practice is dangerous to the owner or the service man who is required to wear protective clothing and also, most likely, a self contained air mask in order to comply with exposure limits for the mineral acids. The strong acid is also corrosive to the treated surfaces and does not clean the entire system. Additional cost would also always be incurred to fill the system with fresh water again.
Several other methods have been suggested in the prior art but all have disadvantages. For example, Hamilton in U.S. Pat. No. 4,906,384 employs a strong mineral acid treatment of the water in the vessel to remove scale deposits, without draining the water, at a level at which the total alkalinity is zero or a pH of 4.6 (Col. 3, line 5). Because the relatively dilute acid reacts slowly with the scale and is thereby consumed, the total alkalinity must continually be monitored and additional acid added, in order to maintain the pH at 4.6 to achieve the descaling and stain removal. If, for example, a swimming pool is badly scaled and stained, Hamilton's method would require considerable manpower and chemical testing to maintain the pH at 4.6 since the mineral acid added would be consumed by the relatively large amounts of scale and stain present.
Hamilton also suggests that the acidification treatment employ a metal sequestering and/or chelating agent such as ammonium salts or ethylenediamine tetraacetic acid (EDTA). By definition, sequestering agents react with certain metal ions to form coordination complexes. By definition, chelating agents react with metal ions to form a ring structure and strongly hold the metal ion through coordination bonding. Water scale ions, such as calcium, magnesium, iron and copper, may be changed in solution, by reacting with a chelating agent, from a positive ion to a complex negative ion. It is well known that this reaction is an equilibrium reaction and that chelation of the metal ion is less effective in acidic solution than in basic solution. Use of expensive chelating agents in acidic solution is not the most economical approach to scale and stain removal. In commercial literature, Hamilton also recommends that the circulating pump on the system be shut down during treatment. Therefore his method does not clean the entire system but only the main body of water.
Similarly, Kisner in U.S. Pat. No. 5,108,514 teaches a method of removing stains, scale and calcium deposits from the interior surfaces of swimming pools without draining the water by treating only the pool surfaces with a concentrated inorganic or mineral acid in combination with a strong metal chelating agent while the pool pump and circulation system is not operating. Again, Kisner does not clean the entire system and also employs the expensive chelating agent in an inefficient manner by using a concentrated mineral acid mixture. Also, the Kisner method employs a tedious means of applying the treating solution only to the surface of the swimming pool and requires additional man hours and the prolonged exposure to and the additional handling of hazardous strong mineral acid.